1. Field of the Invention
The present invention relates to a semiconductor device, particularly to a sense amplifier used in a semiconductor device especially including a thin film transistor.
2. Description of the Related Art
In a semiconductor memory apparatus, a sense amplifier is used for reading stored data. The construction and the operation of an example of a conventional sense amplifier will be described with reference to FIG. 26. In FIG. 26, gates of NMOS transistors MN1 and MN2 are connected to a first input terminal IN1 and a second input terminal IN2, respectively, corresponding to a pair of bit lines. Both sources of the PMOS load transistors MP1 and MP2 are connected to a power source VDD. The PMOS load transistors MP1 and MP2 have a common gate, which is further connected to a drain of the MP1. The drain of the MP1 is connected to the drain of the MN1. The drain of MP2 is connected to the drain of the MN2 and an output terminal OUT. The MN1 and the MN2 have a common source, which is connected to the drain of an NMOS transistor MN3 functioning as a direct current source. The gate and the source of the MN3 are connected to a bias power source V_BIAS and a power source GND, respectively. The sense amplifier shown in FIG. 26 is included in a current mirror type differential amplifier.
In a semiconductor memory apparatus, when stored data is read out, differential potentials, which are opposite in height, occur in a pair of bit lines depending on whether the data is “Hi” or “Lo”. The sense amplifier shown in FIG. 26 detects a small potential difference of signals in bit line sent to the first input terminal IN1 and the second input terminal IN2. Then the sense amplifier amplifies and output the result. In other words, when the potential of IN2 is larger than the potential of the IN1, the output terminal OUT outputs “Lo”. On the other hand, when the potential of the IN2 is smaller than the potential of the IN1, the output terminal OUT outputs “Hi”. In this way, the sense amplifier is used for reading data stored in the semiconductor memory apparatus.
Recently, an active matrix type image display apparatus using an image display device, especially, a thin film transistor (called TFT hereinafter) having a semiconductor thin film on a glass substrate, has been widely spread. The active matrix type image display apparatus (called image display apparatus, hereinafter) using a TFT has hundreds and thousands to several millions TFT's arranged in matrix and controls charges of pixels. Furthermore, a TFT technology (such as polysilicon TFT technology) is evolving recently in which not only pixel TFT is used to construct a pixel but also TFT's are used for forming a drive circuit, a memory circuit, a control circuit and even CPU at the same time.
However, according to the current TFT technology, the variation in characteristic of transistors is larger than that of the technology in which at least one transistor is formed on a single crystal Si substrate. This means that the circuit in the conventional example shown in FIG. 26 is difficult to use in the present TFT technology. For example, it is assumed that threshold values of the NMOS transistors MN1 and MN2 are 1.0 V and 1.5 V, resulting in a difference of 0.5 V. When the potential of the second input terminal IN2 is 0.2 V larger than the potential of the first input terminal IN1, the output OUT should be “Lo”. However, in reality, the output OUT is “Hi”, resulting in a wrong operation. This is critical when the conventional circuit is used as a read circuit for a dynamic random access memory (DRAM).
When the conventional circuit is used as a read circuit for a static random access memory (SRAM), the potential difference between the input terminals is increased over time. At last, the potential difference absorbs the variation in threshold values of the NMOS transistors MN1 and MN2. Thus, the possibility of causing the wrong operation is decreased. However, the large input potential difference absorbing the threshold variation takes time to obtain. As a result, the reading time becomes longer.